This invention relates to a process for producing a liquid hydrocarbon product by a Fischer Tropsch process.
Although the Fischer Tropsch synthesis has been known since 1923, it has failed to gain widespread commercial use due to the disappointing performance of those process plants which have already been constructed and to the high investment demands required for developing more effective systems. Only in countries such as South Africa, where unique economic factors come into play, has the process achieved any kind of commercial significance.
The Fischer Tropsch synthesis attracts interest because, in combination with other processes, it may be used to convert the large supplies of natural gas which are found in remote locations of the world to usable liquid fuel. The synthesis involves the conversion of synthesis gas, i.e. a gas containing hydrogen and carbon monoxide (which can be obtained by conversion of natural gas), to a liquid hydrocarbon product using a suitable catalyst. The specific reactions taking place, and hence the composition of the end product, depend upon the reaction conditions. These include the ratio of hydrogen to carbon monoxide and the catalyst used. Generally the reactions taking place may be depicted as follows:(2n+1)H2+nCO→CnH2n+2+nH2O(n+1)H2+2nCO→CnH2n+2+nCO22nH2+nCO→CnH2n+nH2OnH2+2nCO→CnH2n+nCO2Byproducts of this reaction include gaseous hydrocarbons, such as methane and ethane.
Suitable catalysts for the synthesis can be found amongst the Group VIII metals. There has been much interest in developing and modifying suitable catalysts in an attempt to improve the commercial viability of the Fischer Tropsch synthesis. Thus U.S. Pat. No. 6,100,304 describes a palladium promoted cobalt catalyst providing a significant activity enhancement comparable to effects seen with rhodium promoted cobalt catalysts. In U.S. Pat. No. 6,087,405 it is stated that Fischer Tropsch synthesis conditions, in particular use of relatively high water partial pressures, can lead to weakening of the catalyst resulting in the formation of fines in the reaction mixture. Catalyst supports are described which are comprised primarily of titania incorporating both silica and alumina which have increased strength and attrition resistance qualities when compared to previous catalyst supports. U.S. Pat. No. 5,968,991 describes a Fischer Tropsch catalyst comprising a titania solid support impregnated with a compound or salt of an appropriate Group VIII metal, a compound or salt of rhenium and a multi-functional carboxylic acid. The multi-functional carboxylic acid acts to facilitate distribution of the compound or salt of the Group VIII metal in a highly dispersed form, thus reducing the amount of rhenium required to produce both dispersion and reduction of the metal. U.S. Pat. No. 5,545,674 teaches a supported particulate cobalt catalyst formed by dispersing the cobalt as a thin catalytically active film upon the surface of a particulate support such as silica or titania. U.S. Pat. No. 5,102,851 discloses that the addition of platinum, iridium or rhodium to a cobalt catalyst supported on an alumina carrier, without additional metal or metal oxide promoters, provides a higher than expected increase in the activity of the catalyst for Fischer Tropsch conversions. U.S. Pat. No. 5,023,277 describes a cobalt/zinc catalyst which is said to be very selective to hydrocarbons in the C5 to C60 range and enables the synthesis to be operated under conditions of low carbon dioxide make and low oxygenates make. U.S. Pat. No. 4,874,732 teaches that the addition of manganese oxide or manganese oxide/zirconium oxide promoters to cobalt catalysts, combined with a molecular sieve, results in improved product selectivity along with enhanced stability and catalyst life.
With a view to further improving the viability of the Fischer Tropsch synthesis aspects of slurry processes have also been investigated, such as product removal, catalyst rejuvenation, catalyst activation, gas distribution and adaptation of reactor designs. U.S. Pat. No. 6,069,179 comments that a problem associated with slurry reactors used to effect the Fischer Tropsch synthesis is separation of the catalyst from the product stream in a continuous operation. This problem is addressed by providing a pressure differential filter member. U.S. Pat. No. 6,068,760 tackles the same problem by feeding a portion of the slurry through a dynamic settler which enables clarified wax to be removed from the slurry which is then returned to the reactor. U.S. Pat. No. 5,900,159 employs a method of degasifying the slurry and passing it through a cross-flow filter in order to separate the product from the solid catalyst. U.S. Pat. No. 6,076,810 comments that problems commonly encountered in slurry reactors, amongst others, are gas injector plugging and catalyst particle attrition. A proposed solution is provided by means of a gas distribution grid which includes a plurality of gas injectors horizontally arrayed across a plate which is otherwise gas and liquid impervious. U.S. Pat. No. 5,973,012 proposes to rejuvenate deactivated Fischer Tropsch catalyst by subjecting a portion of the slurry from the reactor to degasification, contacting the degasified slurry with a suitable rejuvenating gas and then returning it to the reactor. U.S. Pat. No. 4,729,981 relates to the provision of both promoted and unpromoted, supported cobalt and nickel catalysts activated by reduction in hydrogen, followed by oxidation with an oxygen-containing gas and ultimately, a second reduction in hydrogen. Such activation results in improved reaction rates regardless of the method of preparation of the catalyst. U.S. Pat. No. 5,384,336 teaches a multi-tubular configuration for a bubble column type reactor, while U.S. Pat. No. 5,776,988 proposes an ebulliating reactor, to obtain enhanced heat transfer through the system and the prevention of hot spots.
Reviews of Fischer Tropsch reactor designs have been published by Iglesia et al., Advances in Catalysis, Vol. 39, 1993, 221-301 and Sie and Krishna, Applied Catalysis A General, 186, (1999), 55-70.
There are several different configurations of Fischer Tropsch reactors, including fixed bed multitubular reactors, vapour phase fluidised bed reactors and slurry or three phase reactors.
In general, slurry or three phase reactors have the advantage that it is possible to use small catalyst particles without the occurrence of high pressure drop problems which feature in fixed bed reactors. Moreover use of small catalyst particles has been shown to reduce the yield of methane as demonstrated by Iglesia et al., Advances in Catalysis, Vol. 39, 1993, 221-301.
In general, designs for Fischer Tropsch reactors have adopted a “long and thin” construction as this has proved to be a suitable design to allow sufficient heat removal and allows realization of the benefit of plug flow conditions. In plug flow systems the catalyst is stationary relative to the flow of the gas and liquid phases. As the feed stream enters the reactor the reactants begin to convert to products and this conversion continues as the feed stream continues through the reactor. A consequence of this is that the concentration and partial pressure of the reactants decrease as the feed stream passes through the reactor and the concentration of product increases, resulting in a drop in the driving force for the reaction. The required volume of the reactor for most straight-forward processes, where the rate of reaction is dependent upon the concentration of the reactants, can be reduced when compared to other systems, therefore enabling a significant cost saving to be made in the construction of the plant.
Benson et al, IEC, vol 46, No 11, November 1954, describe an oil circulation process for the Fischer Tropsch synthesis in which the oil circulation cools the reaction product. The process employs a reactor with a height to diameter ratio of 12 or more and gas is bubbled up through the liquid phase at a superficial velocity below 0.03 m/sec in order to avoid catalyst disintegration.
Fully back mixed reactors (CSTR) are a standard design option for laboratory scale reactors for use with many different processes, including the Fischer Tropsch synthesis. These laboratory scale reactors employ an agitator to provide mixing and solid distribution, and are used to investigate reaction kinetics under uniform conditions. The rate of conversion of reactants to products, along with the product selectivity, depends upon the partial pressure of the reactants that are in contact with the catalyst. The mixing characteristics of the reactor determine the gas phase composition which is critical to catalyst performance. In fully back mixed reactors (CSTR) the composition of the gas and liquid phases is constant throughout the reactor and the gas partial pressure provides the driving force for the reaction, thus determining the conversion of the reactants.
U.S. Pat. No. 5,348,982 compares the fully back mixed reactor (CSTR) system with that of the plug flow system and concludes that the productivity of the fully back mixed reactor (CSTR) system will always be lower than the productivity of the plug flow system for reactions with positive pressure order kinetics. This is because the gas phase reactant concentrations providing the driving force for the reaction differ significantly between the two systems. The reactant concentration, and hence reaction rate, at any point in a fully back mixed reactor (CSTR) system, will always correspond to the outlet conditions. In a plug flow system, as the reactant concentration steadily decreases between the inlet and outlet, the rate of reaction is the integral of the rate function from inlet to outlet. U.S. Pat. No. 5,348,982 proffers a slurry bubble column which addresses the problems associated with the scale-up of laboratory practices on a commercial scale. The bubble column is operated under plug flow conditions and employs a gas up-flow sufficient to achieve fluidisation of the catalyst, but back mixing of the reactants is minimised.
U.S. Pat. No. 5,827,902 describes a process for effecting the Fischer Tropsch synthesis in a multistage bubble column reactor paying particular attention to the problem of thermal exchanges, which is a significant problem in systems utilised for exothermic reactions such as the Fischer Tropsch synthesis.